There are two main sorts of grey gold alloys on the market: alloys in which the whitening metal for the gold is nickel, and those where this metal is palladium.
Although less commonly used in jewelry, because of their allergenic properties, alloys of nickel can still be used in watchmaking for components that never come into contact with the skin. Further, the low material cost of nickel compared to palladium makes such alloys advantageous for these watchmaking applications.
However, each of these gold alloys has drawbacks.
Indeed, although these gold-nickel alloys exhibit very low chromaticity, which makes them very attractive for their relative whiteness, they can only have one shaping mode—lost-wax casting—since in the annealed state they have a high hardness, typically greater than 260 HV for an 18 carat gold alloy with 21% by mass of nickel. This hardness means that they are difficult to cold work and are therefore unsuitable for the working conditions of jewelers and manufacturers of external timepiece parts, such as watch cases, hands, dial appliques, etc., who are the main users of such alloys. It was noted, in particular, during tests on these gold-nickel alloys, that they are susceptible to cracking during cold drawing operations and during heat/hardening treatments, and during recrystallization annealing after deformation, particularly when the nickel content exceeds 5% by mass.
It will also be noted that alloys with a relatively low gold content, typically 9 carat gold alloys, are susceptible to cracking corrosion under stress, as described, for example, by B. Neumeyer in the publication entitled “A facile chemical screening method for the detection of stress corrosion cracking in 9 carat gold alloys”, Gold Bulletin, volume 42, No. 3 2009. This document discloses, in particular, at page 75, Table 1, a 10 carat gold alloy, containing 10.3 to 20% Ni, 25.2 to 41.6% Cu, and 4.3 to 13.1% Zn, which is usable as a wire or as a sheet, and having one preparation method that includes several rolling steps, and annealing in an N2 and H2 atmosphere at 800° C.
Palladium-gold alloys are expensive due to the price of palladium, and because a substantial amount must be added to the alloy to obtain a whitening effect. Further, although the hardness of palladium-gold alloys, typically 120 HV, certainly allows for satisfactory cold working, it is insufficient to meet the necessary requirements for the manufacture of external timepiece parts.
It is difficult to produce nickel-gold alloys by rolling: numerous rolling passes produce undesirable metallurgical defects, thus the malleability of the alloy decreases as the rolling process advances. Unfortunately, recrystallization anneals, performed to restore properties, homogenize the alloy, with hardening, by solution treatment of the nickel, which is unfavourable for subsequent deformations.